EQUITY INVESTMENTS (Details) - USD ($) $ in Millions					1 Months Ended			3 Months Ended								7 Months Ended	12 Months Ended
	Jan. 06, 2020	Nov. 01, 2019	Sep. 01, 2017	Jun. 01, 2017	Aug. 31, 2019	Jun. 30, 2019	Apr. 30, 2006	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Aug. 01, 2017
EQUITY INVESTMENTS
Equity Earnings								$ 45.0	$ 31.0	$ 30.0	$ 54.0	$ 44.0	$ 34.0	$ 36.0	$ 59.0		$ 160.0	$ 173.0	$ 124.0
Equity Investments								1,098.0				1,196.0					1,098.0	1,196.0
Distributions from Equity Investments
Distributions received from equity investments																	258.0	198.0	145.0
Distributions from equity investments																	200.0	188.0	140.0
Return on investment distribution classified as investing activities																	58.0	10.0	5.0
LIABILITIES AND PARTNERS' EQUITY
Current portion of long-term debt (Note 9)								123.0				$ 36.0					$ 123.0	36.0
Revenues (expenses)
Transmission revenues																		194.0
Operating expenses																		(57.0)
Depreciation																		(29.0)
Financial charges and other																		(14.0)
Net income																		$ 94.0
Northern Border
EQUITY INVESTMENTS
Partnership interest held (as a percent)																	50.00%
Great Lakes
EQUITY INVESTMENTS
Partnership interest held (as a percent)																	46.45%
Amount of difference between the carrying value and the underlying equity in net assets resulting from the recognition and inclusion of goodwill								$ 260.0									$ 260.0
Great Lakes | Minimum
EQUITY INVESTMENTS
Excess of estimated fair value over carrying value (as a percent)												10.00%						10.00%
ONEOK Partners, L.P. | Northern Border
EQUITY INVESTMENTS
Ownership interest (as a percent)								50.00%									50.00%
Northern Border
EQUITY INVESTMENTS
Ownership interest (as a percent)			50.00%
Equity contribution			$ 83.0
Capital contribution to reduce the outstanding balance of revolver debt			$ 166.0
Cash distribution						$ 100.0
Distribution financed by borrowings (as a percent)						100.00%				100.00%
Northern Border | Revolving credit facility
EQUITY INVESTMENTS
Maximum borrowing capacity						$ 200.0				$ 200.0
Great Lakes
EQUITY INVESTMENTS
Total cash call issued to fund debt repayment												$ 10.0			$ 11.0
LIABILITIES AND PARTNERS' EQUITY
Current portion of long-term debt (Note 9)								$ 21.0				21.0					$ 21.0	$ 21.0
Iroquois
EQUITY INVESTMENTS
Ownership interest (as a percent)								49.34%									49.34%
Total cash call issued to fund debt repayment																		7.0
Distributions from Equity Investments
Return on investment distribution classified as investing activities																	$ 8.0	10.0
Limited partners, Distribution declared		$ 28.0
ASSETS
Cash and cash equivalents								$ 43.0									43.0
Other current assets								36.0									36.0
Property, plant and equipment, net								570.0									570.0
Other assets								16.0									16.0
Assets, total								665.0									665.0
LIABILITIES AND PARTNERS' EQUITY
Current liabilities								34.0									34.0
Net long-term debt, including current maturities								317.0									317.0
Current portion of long-term debt (Note 9)								3.0				146.0					3.0	146.0
Other non-current liabilities								20.0									20.0
Partners' capital								294.0									294.0
Liabilities and Partners' Equity, total								$ 665.0									665.0
Revenues (expenses)
Transmission revenues																$ 110.0	180.0
Operating expenses																(32.0)	(58.0)
Depreciation																(17.0)	(29.0)
Financial charges and other																(9.0)	(11.0)
Net income																52.0	$ 82.0
Northern Border
EQUITY INVESTMENTS
Ownership interest (as a percent)								50.00%									50.00%
Equity Earnings																	$ 69.0	68.0	67.0
Equity Investments								$ 422.0				497.0					422.0	497.0
Amortization period of transaction fee							12 years
Transaction fee							$ 10.0
Additional ownership interest acquired (as a percent)							20.00%
Equity contribution																			83.0
Undistributed earnings																		0.0	0.0	$ 0.0
Amount of difference between the carrying value and the underlying equity in net assets resulting from the recognition and inclusion of goodwill								115.0								$ 115.0	115.0		115.0
Distributions from Equity Investments
Distributions received from equity investments																	144.0	83.0	83.0
Return on investment distribution classified as investing activities																	50.0
ASSETS
Cash and cash equivalents								21.0				10.0					21.0	10.0
Other current assets								37.0				36.0					37.0	36.0
Property, plant and equipment, net								989.0				1,037.0					989.0	1,037.0
Other assets								12.0				13.0					12.0	13.0
Assets, total								1,059.0				1,096.0					1,059.0	1,096.0
LIABILITIES AND PARTNERS' EQUITY
Current liabilities								42.0				34.0					42.0	34.0
Deferred credits and other								39.0				35.0					39.0	35.0
Net long-term debt, including current maturities								364.0				264.0					364.0	264.0
Current portion of long-term debt (Note 9)								0.0									0.0
Partners' capital								615.0				764.0					615.0	764.0
Accumulated other comprehensive loss								(1.0)				(1.0)					(1.0)	(1.0)
Liabilities and Partners' Equity, total								$ 1,059.0				1,096.0					1,059.0	1,096.0
Revenues (expenses)
Transmission revenues																	300.0	289.0	291.0
Operating expenses																	(82.0)	(78.0)	(78.0)
Depreciation																	(62.0)	(60.0)	(59.0)
Financial charges and other																	(18.0)	(15.0)	(18.0)
Net income																	$ 138.0	136.0	$ 136.0
Northern Border | Northern Border
EQUITY INVESTMENTS
Ownership interest (as a percent)								50.00%									50.00%
Great Lakes
EQUITY INVESTMENTS
Ownership interest (as a percent)								46.45%									46.45%
Percentage less than of which fair value exceeded carrying value																10.00%			10.00%
Equity Earnings																	$ 51.0	59.0	$ 31.0
Equity Investments								$ 491.0				489.0					491.0	489.0
Equity contribution								4.6			$ 5.1						10.0	9.0	9.0
Undistributed earnings																	0.0	0.0	0.0
Amount of difference between the carrying value and the underlying equity in net assets resulting from the recognition and inclusion of goodwill												260.0						260.0
Distributions from Equity Investments
Distributions received from equity investments																	(59.0)	58.0	35.0
ASSETS
Current assets								72.0				75.0					72.0	75.0
Property, plant and equipment, net								685.0				689.0					685.0	689.0
Assets, total								757.0				764.0					757.0	764.0
LIABILITIES AND PARTNERS' EQUITY
Current liabilities								33.0				26.0					33.0	26.0
Net long-term debt, including current maturities								219.0				240.0					219.0	240.0
Other non-current liabilities								6.0				4.0					6.0	4.0
Partners' capital								499.0				494.0					499.0	494.0
Liabilities and Partners' Equity, total								$ 757.0				764.0					757.0	764.0
Revenues (expenses)
Transmission revenues																	238.0	246.0	181.0
Operating expenses																	(79.0)	(68.0)	(66.0)
Depreciation																	(32.0)	(32.0)	(29.0)
Financial charges and other																	(16.0)	(18.0)	(20.0)
Net income																	111.0	128.0	66.0
Great Lakes | Iroquois
Distributions from Equity Investments
Distributions received from equity investments																	$ 55.0
Iroquois
EQUITY INVESTMENTS
Ownership interest (as a percent)	49.34%			49.34%				49.34%									49.34%				49.34%
Equity Earnings																	$ 40.0	46.0	26.0
Equity Investments								$ 185.0				210.0					185.0	210.0
Equity contribution				$ 710.0	$ 4.0												4.0
Undistributed earnings																$ 0.0	0.0	0.0
Additional consideration on surplus cash				$ 28.0
Distributions from Equity Investments
Distributions received from equity investments																		56.0
Distributions from equity investments	$ 14.0
Return on investment distribution classified as investing activities																	$ 8.0	10.0	$ 5.0
ASSETS
Cash and cash equivalents												80.0						80.0
Other current assets												32.0						32.0
Property, plant and equipment, net												581.0						581.0
Other assets												8.0						8.0
Assets, total												701.0						701.0
LIABILITIES AND PARTNERS' EQUITY
Current liabilities												19.0						19.0
Net long-term debt, including current maturities												325.0						325.0
Other non-current liabilities												14.0						14.0
Partners' capital												343.0						343.0
Liabilities and Partners' Equity, total												$ 701.0						$ 701.0